write_prometheus plugin: Fall back to IPv4 socket if IPv6 fails.

write_prometheus plugin: Fall back to IPv4 socket if IPv6 fails.

Unfortunately, for libmicrohttpd "dual stack" means "enforce IPv6 (but
allow IPv4, too)". That means that on systems without IPv6,
libmicrohttpd'd initialization fails (when the dual stack flag is set).
To work around this limitation we open the socket ourselves and pass it
to libmicrohttpd to use.

Fixes: #2149